Feed large aquatic plants by inserting slow-release fertiliser tablets well below soil level around the base of the plant. Waterlilies are also big feeders. Feed any plants not being divided.
Tidy up plants in the bog garden, and mulch with composted bark (or similar).
Continue dividing or cutting back marginal and bog garden plants if overcrowded.
Divide and replant waterlilies once they show signs of growth.
Contain vigorous pond perennials by planting them in aquatic plant baskets and topping with a layer of gravel to prevent fish stirring up the compost.
Small plants in baskets can be raised up on bricks when first introduced then slowly lowered to the bottom of the pond as they increase in size.
Troubleshooting
Drain and clean neglected ponds if black and sludgy.